Light meters are commonly used in cameras to determine whether there is sufficient light in the scene to produce a properly exposed image. When there is insufficient light in the scene, the camera may utilize a flash to aid in the exposure of the scene. A common feature in cameras is automatic flash. When the light meter indicates that the amount of light in the scene, which is typically an average brightness in the scene, is below a threshold brightness level, an automatic flash circuit initiates the flash. Unfortunately, there are instances when the average brightness in the scene is sufficiently above the threshold because of a bright background, but the subject is dimly lit. This type of exposure is commonly referred to as a back light exposure.
When photographing a dimly lit subject in a scene with a surrounding illumination condition, such as the sun, an incandescent light, or the headlights of, cars at night, the subject is placed in a backlight illumination state. The resulting image of the subject will be under-exposed.
That is, when photographing a backlit subject without backlight compensation, the exposure will be controlled in accordance with the overall luminance of the scene without any allowance for the luminance level of the object, thereby producing an underexposed image of the subject. To compensate for this situation, the camera is provided with backlight compensation.
Some cameras have a backlight compensation feature that is manually set by a user and other cameras have an automatic backlight compensation feature that automatically detects backlight conditions and automatically adjusts the iris to improve the resulting image. Unfortunately, both the manual and automatic backlight compensation offer only tolerable results.
U.S. Pat. No. 5,963,254 describes a prior art backlight compensation technique that performs automatic backlight compensation without need for manually re-designating a backlight compensation area, despite changes in ambient illumination, by selecting the most appropriate backlight compensation area for changes in the ambient illumination and the movement of an object from among a plurality of predetermined backlight compensation areas. Backlight compensation is performed by adjusting the positions and sizes of backlight compensation areas so as to be suitable to the surroundings of the monitoring camera by a user, and automatically or manually selecting the most appropriate backlight compensation area in accordance with changes in illumination and changes in the position of the object, and controlling the iris according to the luminance level of the selected backlight compensation area.
One disadvantage of this approach is that the use of an iris driver and an iris driving signal generator, along with associated adjustment hardware, complicates camera design and raises reliability issues. However, the subject could still need to be illuminated by flash, even if the iris has been adjusted to compensate for back lighting.
U.S. Pat. No. 6,389,232 describes another prior art exposure control technique. This exposure control method and apparatus for a camera has a two-area divided brightness measurement system. A central light value is determined based on a central area brightness of a photographic scene, and a peripheral light value is determined based on a peripheral area brightness of the scene. After determining by comparison between the central and peripheral light values whether the scene is back-lighted or front-lighted, a correction coefficient specific to back-lighted scenes or that specific to front-lighted scenes is read from a memory. An exposure value suitable for a main subject is calculated according a predetermined equation: Es=log.sub.2 {1−(1−.alpha.)2.sup.LVb−LVa}−log.sub.2.alpha.+LVa, where Es represents the exposure value and .alpha. represents the correction coefficient.
One disadvantage of this approach is that it requires a memory to store the correction coefficient and additional logic to implement the above equation. As can be appreciated, the additional required hardware and/or software complicates the camera design and raises reliability issues. However, a flash could still be required to illuminate a central subject.
Based on the foregoing, there remains a need for an exposure compensation method and system that overcomes the disadvantages of the prior art as set forth; previously.